1. Field of the Invention
The present invention generally relates to ceiling structures and, more particularly, to planar ceiling structures including air filtration systems for clean rooms as well as other facilities such as lighting systems, fire extinguishing systems and electrical distribution systems.
2. Description of the Prior Art
Many products such as high-precision machines and electronic components are highly sensitive to contamination from particulate materials often found in the ambient atmosphere. Once manufactured, these products can usually be sealed or protection from contamination otherwise provided. However, during the course of manufacture of these types of products, no such protection can be provided for the products themselves and reliance is placed upon the provision of an environment, commonly referred to as a "clean room" from which contaminants and sources of contaminant material are rigorously excluded.
Such clean rooms generally rely on continual filtering of the air for the removal of any contaminant material which may be present or introduced. In this regard, contaminants may actually be produced in clean rooms during manufacturing processes involving the working or removal of materials from workpieces or condensing or coalescing from vapors. Therefore it is imperative that all air within the clean room be continually circulated and filtered. It is similarly important that contaminants, when produced, be quickly transported to locations where they can be captured and thus restrained from circulating to other areas of the clean room. For this purpose, air is usually circulated through the clean room from the ceiling to the floor, often provided as a raised grid structure, and then returned through ducts to the ceiling. This provides circulation across the shortest dimension of the room and effectively confines contaminants to the area where they are produced. Also, since contaminants are typically more dense than the atmosphere, the rapid transportation of contaminants is enhanced by gravity. The air is filtered at its exit from the ceiling. Distribution of air should also be over as much of the ceiling area as possible to prevent the entraining of air or other anomalies in the ceiling to floor circulation pattern.
However, it is generally necessary to also provide other services such as light and electricity, compressed air, fire extinguishing systems and the like in the clean room in addition to highly filtered air, in order to provide for worker safety and to support the manufacturing process being carried out. These services are collectively referred to as "facilities". The provision of these facilities often can compromise the clean room environment since they usually require penetration of the walls, ceilings and/or floors of the clean room. Also, the materials involved, such as electrical insulation, as well as material of the structural room itself (e.g. the portion of the building structure in which the clean room is constructed) where facilities enter the clean room, can be sources of contaminant material. Further, in large spaces, provision of facilities is most conveniently done at the ceiling. Since any discontinuity, such as holes, in the filters is undesirable, areas of the ceiling must be effectively dedicated to the facilities and are therefore unavailable for maintaining the ceiling to floor air circulation pattern. The provision of lighting is particularly difficult in this regard since lighting must also be evenly distributed in most installations and generally must form part of the ceiling structure to achieve this distribution.
Similarly, it is not desirable to suspend any structure having a significant horizontal area below the ceiling of a clean room since such areas disrupt the desired air flow pattern and may also allow particulate materials to collect. Then, some slight mechanical disturbance may cause collected materials to be dispersed in a relatively large quantity which could contaminate significant areas of the clean room and products being manufactured before trapping can occur. Lighting also presents a further complication in this regard since suspended lighting, especially lighting containing fluorescent tubes, could be mechanically damaged and thus become a further source of contaminants. Additionally, any facilities which are suspended below the ceiling or present a significant area below the ceiling level will interfere with the distribution of light throughout the area.
Another complication is the provision of partitions for forming so-called mini-environments within the clean room. Mini-environments are often necessary either to more fully exclude or contain contaminants for particular areas of the clean room. Different air flow rate is often provided in these mini-environments than is provided for the remainder of the clean room. These mini-environments are provided by partitions which must generally be attached to the ceiling and suspended therefrom for structural reasons. To avoid presenting a significant horizontal area, relatively thin sheets of material are generally preferred. Suspension close to the ceiling is necessary to avoid entraining of contaminated air near the ceiling. However, since the location for such suspension may not be provided in the areas covered by filters, suspension must be made from the same areas dedicated to facilities. In particular, since lighting and lighting fixtures occupy a large portion of this area, partitions interfere with the distribution of light from the fixtures wherever the partitions are located. In short, all of the facilities interfere with each other and the function of the air circulation and filtration system.
It is also known that the highest air quality in clean rooms is provided by systems which employ plenums for the distribution of air. These plenums are preferably constructed from plenum modules which may be generally regarded as box-like structures have a cross-section in the form of an inverted "U" so that the bottoms are opened to receive filter materials and other air flow regulation devices, such as dampers. Plenum modules may be bolted together end-to-end to form one or more plenums or ducts which extend over the area of the clean room. The sides of the plenum modules may also have openings for receiving filters or communicating air between plenum modules.
The plenums or ducts thus formed of plenum modules form the basic structural members of the air distribution system for the clean room and support the filters elements (and any grille-work which may be included as well as the dampers and other air flow control devices). At the same time the plenums provide for containment of the recirculated air within a surface, such as a baked polyurethane coating on at least the interior of the plenum modules, which does not shed contaminants. Therefore, it is undesirable to modify the plenums or plenum modules in any way, such as by drilling, to route facilities therethrough. Dedicated areas used for access to facilities (where plenum modules are omitted) disrupts the desired air flow pattern, discussed above. Further, omission of plenum modules to produce such dedicated areas reduces the structural integrity of the plenum assembly. The alternative would require modification of plenum modules which compromises the air delivery and filtration performance of the plenum system and may also affect the structural integrity thereof.
Additionally, plenums and plenum modules known in the art do not generally provide structures suitable for forming mini-environments, without modification and do not provide for convenient changes of mini-environments. As indicated above, partitions may only be attached to the ceiling at locations not occupied by the air filtration system. Thus, concentration of facilities in dedicated areas limits the locations at which attachment of partitions to the ceiling may be made. At the same time, the concentration of facilities reduces the regularity of lighting distribution and the limitation of attachment locations for partitions virtually assures interference of the partitions with either the facilities, including lighting distribution, or the air circulation and filtration or both.
Further, since mini-environments are generally formed in order to segregate areas in which differentiated air flow will be used, entrainment of contaminated air often occurs near the ceiling due to the differential air flow rates when the partition locations do not accurately follow the boundaries of the differentiated air flow areas. Particularly since air flow distribution will be disrupted where facilities are concentrated, it is virtually guaranteed that entrainment of contaminated air will occur and will be aggravated by the suspension of partitions from those areas. Support of partitions only from the floor does not provide a solution since the upper edges will then be unstable in a lateral direction and their locations uncertain. Further, a regular clearance from the ceiling and the air inlets may also increase entrainment of contaminated air.